1. Field of the Invention
The present invention relates to connections of the kind commonly used in the oil industry and more particularly to a sealing system for sealing the connection between two members such as cylindrical or tubular members.
2. Background Art
The present invention is applicable broadly for use in joining tubular members, pipes, casing, tubing, wellheads, couplings, and the like, and has particularly been developed for use in oilfield tubular connections. Various forces are applied to an oilfield connection as for example, interior pressure, exterior pressure, tension, compression, bending, torsion and non-axi-symmetric forces. It is important that the oilfield connection maintain a seal, preferably a metal-to-metal seal, as various forces and combinations thereof are applied to the connection. Oilfield connections between tubular members, such as downhole pipe, experience the most severe and demanding conditions.
One type of sealing system is disclosed in U.S. Pat. Nos. 5,415,442 and 5,462,315, incorporated herein by reference. Those patents disclose a stabilized center-shoulder-seal tubular connection. This connection is stabilized using at least one run-out section of threads adjacent the center-shoulder seal configuration of each connection member. The run-out section of threads is typically used in combination with a run-in section of threads on the corresponding connection member. The stabilization of the connection at its center provides a stabilization of the seals at the center shoulders.
The center-shoulder seal of the '442 and '315 patents includes pin and box members each having a center shoulder configuration and a ramp. Each center shoulder configuration includes an annular groove forming an adjacent annular shoulder. The annular shoulder forms a ring surface and the groove forms an annular undercut surface. An exterior cylindrical ramp surface is formed on the exterior of the shoulder and a cylindrical center surface is formed on the interior of the shoulder. An exterior sealing surface, in the form of a bevel or chamfer, extends between the ring surface and the exterior cylindrical ramp surface. The undercut surface forms an interior cylindrical ramp surface and an interior sealing surface, in the form of a bevel or chamfer, extending between the interior cylindrical ramp surface and the undercut surface.
Upon the assembly of the center shoulder seal disclosed in the '442 and '315 patents, the ramps on the pin and box members engage the interior and exterior cylindrical ramp surfaces to align and guide the center shoulder configurations together. As the assembly progresses, a metal-to-metal seal is created as the interior and exterior sealing surfaces engage. The engagement of the interior and exterior sealing surfaces causes the annular shoulders to bend inwardly to close the gap between the cylindrical center surfaces on the annular shoulders thereby producing a zero clearance seal. However, the latter may not seal if there is substantial clearance between the cylindrical center surfaces prior to the bending motion. The sealing engagement of the cylindrical center seals is merely a consequence of the interior and exterior sealing surfaces bending the shoulders inwardly.
Upon the final power tightening (torquing) of the assembled connection, the ring surfaces contact the undercut surfaces thereby creating two additional zero clearance seals. Thus, a total of at least four and possibly five seals are created during makeup: two seals at the interior and exterior sealing surfaces, and three zero clearance seals, one at the cylindrical center surfaces, and two at the ring and undercut surfaces. Typically, a thread compound is used on the surfaces of the threaded connection to assist the intended zero clearance seals to bridge any surface defects or machine tolerances which result in a clearance between metal surfaces of less than about 0.005 inches. In the sealing system of the '442 and '315 patents, the sealing system includes a cylindrical center seal, the interior and exterior seals, and the shoulder/undercut seals.
The sealing system disclosed in the '442 and '315 patents is located at the center of the body of the connection and thus the sealing system is encapsulated in a static field which is isolated from the various loads placed on the connection which might otherwise tend to unseal the seals. The interior and exterior seals caused by the sealing engagement of the interior and exterior sealing surfaces cause the annular shoulders to bend inwardly and preferably cause the mating cylindrical center surfaces to form a sealing engagement, i.e. a zero clearance seal.
One disadvantage of prior art metal-to-metal sealing systems is that the absolute positions of the metal-to-metal sealing surfaces must be maintained with respect to the pin and box members. Prior art sealing systems must have sealing surfaces that have absolute diametrical positions with respect to the pin and box members to achieve the desired contact pressure between the sealing surfaces so as to form a seal. The absolute positions for prior art sealing surfaces on each member are measured from a reference point on each of the pin and box members so that upon connecting the pin and box members, the sealing surfaces have sufficient contact pressure so as to form metal-to-metal seals. Typically, the reference point is the shoulder in the box member which registers with the terminal end of the pin member.
The nominal dimensions of the sealing surfaces are the design dimensions. Each nominal dimension has a range of tolerances so as to form a band or range of dimensions which are plus or minus the nominal dimension. To ensure sealing engagement, the actual dimension of the sealing surface must fall within the range of tolerances of the nominal dimension. Therefore, to achieve sealing engagement, the actual dimensions of the sealing surfaces must fall within the tolerance band so as to have the required absolute positioning with respect to the pin and box members. Failure to have the proper absolute positioning of the sealing surfaces with respect to the pin and box members will prevent the sealing surfaces from forming the desired metal-to-metal seal.
The absolute dimensions of the sealing surfaces require a very narrow band of tolerances since even a few thousandths of an inch of interference can greatly affect the sealing contact between the sealing surfaces. Thus, the diameter and length of the sealing surfaces from the reference point must be maintained within the band of tolerances to ensure the establishment of a metal-to-metal seal.
The sealing surfaces on the pin and box members are typically cut using a computer numerical control (CNC) machine. The CNC machine supports a carbide tool insert which cuts the sealing surfaces. Typically, the sealing surfaces of a prior art sealing system are cut by making multiple passes of the carbide tool insert to produce multiple sealing surfaces, all having an absolute position with respect to the pin and box members. The CNC machine and operator must control the carbide tool in four directions, right and left of the reference point and in and out with respect to the reference point. However, multiple passes or strokes of the carbide tool insert for cutting multiple sealing surfaces allows greater variances in the diametrical dimensions of the sealing surfaces causing greater probability of having an actual dimension which falls outside the tolerance range.
The large number of sealing surfaces exacerbates the objective of achieving close tolerances to ensure metal-to-metal sealing engagement. For example, the sealing system of the '442 and '315 patents requires that seven different sealing surfaces be controlled on each complimentary pin and box member of the connection. Not only must the absolute position of the diametrical dimensions of the cylindrical surfaces be maintained but also the tolerances of the various relief angles and radiuses.
Typically, the absolute positioning of the sealing surfaces must be maintained to tolerance of at least 0.002 inches and preferably 0.001 inches for premium connections. Whenever these tolerances are not achieved, the connection must be recut to meet specifications which adds further cost and expense. This is particularly a problem where tolerances must be held to .+-.0.001 inches since such close tolerances are difficult and costly to maintain for the reasons previously described.
The present invention overcomes the deficiencies of the prior art.